212Pb: Production Approaches and Targeted Therapy Applications

Over the last decade, targeted alpha therapy has demonstrated its high effectiveness in treating various oncological diseases. Lead-212, with a convenient half-life of 10.64 h, and daughter alpha-emitter short-lived ²¹²Bi (T_1/2 = 1 h), provides the possibility for the synthesis and purification of complex radiopharmaceuticals with minimum loss of radioactivity during preparation. As a benefit for clinical implementation, it can be milked from a radionuclide generator in different ways. The main approaches applied for these purposes are considered and described in this review, including chromatographic, solution, and other techniques to isolate ²¹²Pb from its parent radionuclide. Furthermore, molecules used for lead’s binding and radiochemical features of preparation and stability of compounds labeled with ²¹²Pb are discussed. The results of preclinical studies with an estimation of therapeutic and tolerant doses as well as recently initiated clinical trials of targeted radiopharmaceuticals are presented.

The effects of ionizing radiation on domestic dogs: a review of the atomic bomb testing era

Dogs were frequently employed as laboratory subjects during the era of atomic bomb testing (1950–1980), particularly in studies used to generate predictive data regarding the expected effects of accidental human occupational exposure to radiation. The bulk of these studies were only partly reported in the primary literature, despite providing vital information regarding the effects of radiation exposure on a model mammalian species. Herein we review this literature and summarize the biological effects in relation to the isotopes used and the method of radionuclide exposure. Overall, these studies demonstrate the wide range of developmental and physiological effects of exposure to radiation and radionuclides in a mid‐sized mammal.

A physiological skeletal model for radionuclide and stable element biokinetics in children and adults

A physiological skeletal model (PSM) is described that represents the skeletal uptake, retention, and clearance of both bone-surface-seeking and bone-volume-seeking radionuclides and stable elements. A key objective of the PSM is to model the higher skeletal growth and bone turnover in infants and children (compared to adults) in order to account for their greater uptake and cancer risk from bone-seeking contaminants such as lead and plutonium. The PSM is a compartmental model that allows for the incorporation of organic and inorganic material in the bone volume via quiescent bone surfaces, forming bone surfaces and the lacuno-canaliculi system. The model uniquely incorporates a tertiary phase of mineralization via bone fluids. The PSM's structural concepts and biokinetic parameters--such as realistic mass transfers, organ and tissue masses, and bone remodeling half-times--are selected mainly on the basis of physiological and anatomical criteria. For brevity, model parameter values are evaluated for adults only. The PSM is an improvement on existing skeletal models that are based more on compartment structures and pathways that rendered good fits to biokinetic data rather than on being anatomically and physiologically accurate.

227Th-EDTMP: a potential therapeutic agent for bone metastasis

The biodistribution of 227Th-EDTMP and retention of its daughter nuclide 223Ra were examined. 227Th-EDTMP was found to show high uptake and long-term retention in bone. The clearance of 227Th-EDTMP from blood and soft tissues was rapid and the femur-to-tissue uptake ratios reached more than 100 within 30 min for all tissues except the kidney. Seven and 14 days after injection of 227Th-EDTMP, the retention index of 223Ra in bone showed high values, and the differences between these time points were not significant. Therefore, 227Th-EDTMP is a potential radiotherapeutic agent for bone metastasis.

Does longevity in beagles injected with bone-seeking radionuclides depend upon radiation dose in the absence of known radiation effects?

Regression analyses of longevity as a function of skeletal radiation dose among groups of beagles injected with 226Ra, 228Ra, 228Th, 241Am, 90Sr or monomeric 239Pu suggested that at low doses and dose-rates (those at which induced effects are low), age at death seems to be independent of dose when animals dying with specific radiation effects were excluded, although longevity does appear to be a function of dose when animals dying with established radiation effects and at all doses were included. We conclude tentatively that, for mammals receiving skeletal dose from bone-seeking radionuclides at low doses and low dose-rates, longevity may not be dependent upon skeletal radiation dose in the absence of radiation-induced malignancies or other radiation effects.

Effective thresholds for induction of skeletal malignancies by radionuclides

Our analysis of data from the beagle project completed at the University of Utah has provided some comparisons that appear to be useful in testing the model proposed by Raabe of effective thresholds for induction of skeletal malignancy by bone-seeking radionuclides in beagles. Raabe's model predicted that cumulative skeletal doses of less than about 0.9 to 1.4 Gy from alpha emitters or 28 to 70 Gy from beta emitters deposited in the skeleton require a long enough time for bone cancer expression that the dog's natural lifespan would be exceeded before the tumor appeared. Results from the Utah beagle project seem to confirm these projections for 226Ra, 228Ra and, perhaps, for 90Sr. The lowest doses at which malignant bone tumors were observed in animals injected with these radium isotopes were about 0.9 Gy (226Ra) and 3 Gy (228Ra). For the beta emitter, 90Sr, the lowest doses at which bone tumors were seen were about 18, 50, and 70 Gy with an expectation for naturally occurring tumor of about one. Twenty-six of the two hundred and thirty-three Utah beagles given monomeric 239Pu that developed skeletal malignancies had doses between 0.02 and 0.51 Gy (80 of these dogs had skeletal doses of less than 0.9 Gy). Three dogs of 54 given 241Am with doses lower than 0.9 Gy had bone tumors at 0.23, 0.56, and 0.88 Gy with the expectation of about one naturally occurring case. For 25 animals injected with 228Th at skeletal doses below 0.9 Gy, one bone tumor dog had a dose of about 0.4 Gy, and the expectation of a dog with natural tumor among the group was only about 0.38. Five beagles of 74 given 224Ra with resulting doses of less than 0.9 Gy died with skeletal malignancy at 0.32 Gy or less with an expectation for non 224Ra induced tumor of about one. It appears that Raabe's proposal might be confirmed for some but not all of the radionuclides used in the Utah studies. Models presented in earlier papers by Raabe provide results that are somewhat different from his recent abstract and compare more favorably with those cited herein for Utah dogs. Re-examination of our data for these analyses has suggested a novel concept for calculation of carcinogenic dose to endosteal bone surfaces.

Does body size contribute to sensitivity of bone tumor induction by radionuclide exposure?

Investigation of a possible increase in sensitivity to occurrence of radionuclide-induced skeletal malignancy with increasing body size was analyzed among 358 beagles injected as young adults with either 226Ra or monomeric 239Pu and maintained for their lifespans. Corresponding analyses were performed for about 240 other beagles injected as young adults with 90Sr, 228Ra, or 228Th. Body masses at the time of injection ranged between about 5.6 and 16 kg. Logistic regression analysis using body mass and cumulative skeletal radiation dose as the independent variables indicated that there could not be established a dependency of tumor occurrence upon body mass, although skeletal dose was found to be significantly correlated with occurrence of bone cancer. Regression analysis indicated that for any dosage group there could not be established a correlation between body mass and skeletal dose. Each dosage group having similar injected kBq kg(-1) for each nuclide was divided into 2 subgroups of equal size, one containing the less massive dogs and the other containing the more massive dogs. These subgroups within a roughly uniform value of skeletal dose-rate were compared by Fisher's Exact Test, and the less massive subgroups were combined within each nuclide for an additional, separate analysis against the combined more massive subgroups using the same method. In only one instance (the dosage group given 3607 kBq 90Sr kg(-1)) was there indicated a substantially greater tumor occurrence among dogs in the more massive subgroup (p = 0.061). However, for the group given 0.382 kBq 239Pu kg(-1) there was indicated a significant difference between subgroups, but the effect was exactly opposite to that found for the highest level 90Sr dogs in that the less massive subgroup had a higher relative tumor occurrence than the most massive (p = 0.042). For all groups with a p-value < 0.10, a possible correlation was investigated between survival and body mass at injection (since bone tumor occurrence might be a function of longevity), but a significant relationship could not be determined. No significant differences could be established between the combined more massive and the combined less massive subgroups for any radionuclide. We conclude that, for the conditions in our experiment, relative size within a species does not contribute importantly to the sensitivity (lifetime occurrence) for induction of skeletal malignancy.